A membrane filtering device is known which is constructed in such a manner that a plurality of membrane elements are disposed on a straight line, and the above core tubes of adjacent membrane elements are connected with an interconnector (connecting section). The plurality of membrane elements that are connected in this manner are housed, for example, in a pressure-resistant vessel formed of resin, and are treated as one membrane filtering device (for example, refer to Patent Document 1).
A membrane filtering device of this kind is generally used for obtaining purified permeated water (permeated liquid) by filtering raw water (raw liquid) such as waste water or sea water. Particularly in a large-scale plant or the like, numerous membrane filtering devices are held by a rack referred to as a train, whereby management of processing characteristics (pressure, water quality and water amount of the permeated water, and the like) is carried out train by train.
However, when the management of processing characteristics is carried out train by train as described above, it is difficult to specify the location of an inconvenience when the inconvenience occurs in the membrane element or the connecting section of only a part of the membrane filtering devices among the numerous membrane filtering devices that are held by a train, thereby raising a problem in that a lot of labor will be required in the specifying work.
Also, with the construction in which the numerous membrane filtering devices equipped with the plurality of membrane elements are held by the train as described above, the fouling degree of a separation membrane and the load imposed when the raw liquid is filtered by the separation membrane will differ depending on the position of each membrane filtering device in the train or the position of each membrane element within each membrane filtering device. Therefore, in replacing the membrane elements, optimization of the arrangement and combination of the membrane elements is carried out so that an optimum processing performance can be eventually exhibited in the whole train, by housing new membrane elements and still usable membrane elements in a suitable combination within the pressure-resistant vessel. However, in the current situation, the optimization is carried out only based on the term of use, so that it is not possible to say that a sufficient optimization is carried out.
Further, the determination of whether a maintenance such as cleaning or replacing of the membrane elements is to be carried out or not is made based on the processing characteristics for each train, so that there is a case in which the maintenance is not necessarily carried out suitably according to the position or the term of use depending on the membrane elements. In other words, depending on the cases, there is a case in which some membrane elements are in a state where it is too late to perform the maintenance or a case in which the maintenance is carried out at a stage earlier than needed.
In order to cope with the aforementioned problems, the following can be made by using a technique such as disclosed in Patent Document 1 described above. Specifically, for each membrane element, the data related to the aforesaid processing characteristics are stored in advance in a wireless tag (RFID tag) disposed in the membrane element, and the data are read out from each wireless tag, whereby management of the processing characteristics can be carried out for each membrane element. However, even in a case in which the management is carried out based on only the data stored in advance in such a wireless tag, the state of each membrane element sometimes changes time by time, so that it is not possible to say that the precision of management is sufficient. Thus, when the state of each membrane element can be detected in real time, the management can be carried out with a better precision.
Therefore, a method is known in which the state of each membrane element is detected in real time with use of a sensor or the like (for example, refer to Patent Document 2). Patent Document 2 discloses that a flow rate sensor, an electric conductivity sensor, or the like is provided in a plurality of membrane elements.